1. Field of the Invention
The present invention relates generally to a patterning method of a chemical amplification type resist film. More specifically, the invention relates to a patterning method of a chemical amplification type resist film for a far ultraviolet radiation, to be formed on an antireflection film.
2. Description of the Prior Art
In a lithographic process as one of a fabrication process of a semiconductor element, at first, a silicon oxide film, a BPSG film, a silicon nitride film, a polycrystalline silicon film, various silicide film, a metal film such as an aluminum or an aluminum alloy film and the like, are formed on a silicon substrate. Next, a resist film is applied on the film. The resist film is exposed in a desired pattern, and then developed. Subsequently, the film which should be patterned is etched with taking the resist film patterned into the desired pattern as a mask.
In such lithographic process, if the film to be patterned is the polycrystalline silicon film, the silicide film, the metal film such as aluminum or aluminum alloy film and the like, reflection of a light at an interface between the resist film and the film to be patterned becomes large. Accordingly, the shape of the pattern of the resist film after development is significantly degraded by reflection of the light. Therefore, in the prior art, in order to reduce reflection index at the interface between the film to be patterned and the resist film, various methods for forming an antireflection films between the resist film and the film to be patterned have been proposed (see Japanese Unexamined Patent Publications (Kokai) Nos. Showa 59-6540, Showa 62-46529 and Heisei 1-241125).
Hereinafter, a fabrication process of the semiconductor device disclosed in Japanese Unexamined Patent Publication No. Showa 59-6540 will be referred to as "first prior art", an etching method of the film to be patterned disclosed in Japanese Unexamined Patent Publication No. Showa 62-46529 will be referred to as "second prior art", and a fabrication process of the semiconductor device disclosed in Japanese Unexamined Patent Publication No. Heisei 1-241125 will be referred to as "third prior art".
FIG. 1 is a section showing the fabrication process of the semiconductor device in the first prior art. An oxide layer 202a is formed on the surface of a semiconductor substrate 201a. A metal film (film to be patterned) 213 of 0.2 to 1 .mu.m thick, is formed on the surface of the oxide layer 202a. Next, a layer semi-permeable to a light to be used in lithography, such as an antireflection film 221a of silicon nitride, for example, is grown on the surface of the metal film 213 by way of plasma excited vapor deposition (PECVD) method. Then, a resist film 231a is formed on the surface of the antireflection film 221a.
Subsequently, above these, a photo-mask 261 formed with a desired pattern of light shielding layer 262, is arranged. Next, a light beam 241 is irradiated on the resist film 231a from the upper side of the photo mask 261. Then, the resist film 231a is exposed by a light 241 past through the photo mask 261 in the region where the light shielding layer 262 is not formed.
At this time, a part of the light inciding in the resist film 231a is reflected at the surface of the antireflection film 221a. The remaining light therefore which passes through the antireflection film 221a is reflected at the surface of the metal film 213, and then discharged from the surface of the antireflection film 221a. In the first prior art, the antireflection film 221a is selected so that an intensity of discharged light (reflected light) becomes less than or equal to 30% relative to an intensity of an incident light. Thus, photo-sensitivity of the discharged light relative to the resist film 213 can be lowered to be ignorably low. Accordingly, by the antireflection film 221a, degradation of pattern of the resist film by the reflected light by the metal film 213 can be successfully prevented.
FIG. 2 is a section showing an etching method of the film to be patterned in the second prior art. At first, an oxide layer 202b and a polycrystalline silicon layer (film to be patterned) 214 are sequentially formed on the surface of the silicon substrate 201b. Next, an antireflection film 221b of silicon nitride is formed on the surface of the polycrystalline silicon layer. Then, a resist film 231b is deposit on the surface of the antireflection film 221b. Thereafter, the resist film 231b is selectively exposed by irradiating a light on the resist film 231b in the desired pattern.
In the second prior art, the material of the antireflection film is selected so that a refractive index n of the light in the antireflection film 221b becomes greater than a refractive index n, of the light in the resist film 231b and smaller than a refractive index n.sub.2 of the light in the antireflection film 221b. The refractive index of the light is defined as, the reflected light by the antireflection film 221b and the polycrystalline silicon layer 214 is interfered by incident light to the antireflection film 221b. Thus, amplitude of the discharged light can be made smaller. Accordingly, similarly to the first prior art, degradation of the pattern shape of the resist film 231b by the reflected light by the polycrystalline silicon layer 214, can be successfully prevented.
FIG. 3 is a section showing a fabrication process of the semiconductor device in the third prior art. At first, a field oxide layer 203 is selectively formed on the surface of a silicon substrate 201c. Then, a gate oxide layer 204 is formed on the surface of an element region defined by the field oxide layer 203. Subsequently, on these surfaces, an undercoat layer (film to be patterned) 215 of tungsten silicide is formed. Then, an antireflection film 221c of silicon nitride is formed on the surface of the undercoat layer 215. A resist film 231c is formed on the surface of the antireflection film 221c. Subsequently, similarly to the foregoing first and second prior arts, the resist film 231c is exposed by irradiating light on the resist film 231c in the desired shape.
In the third prior art, material of the undercoat layer 215 and the antireflection film 221c and thickness of the antireflection film 221c and so forth are selected so that an incident light to the antireflection film 221c and a reflected light from the antireflection film 221c and from the undercoat layer 215 cause interference to make the effective reflection index of the undercoat layer minimum. Accordingly, it can be successfully prevent degradation of the pattern shape of the resist film 231c by the reflected light.
On the other hand, the first to third prior art as set forth above, are directed to form a novolac type resist film to be used for a light (g ray) having a wavelength of 436 nm or a light (i ray) having a wavelength of 365 nm. This novolac type resist film shows color degradation property (bleaching characteristics).
However, a chemical amplification type resist film to be used for exposure by far ultraviolet radiation (KrF excimer laser or ArF excimer laser) having wavelength of 248 nm or 193 nm, in either positive type or negative type, generally does not have color degradation property. When the chemical amplification type resist film for far ultraviolet radiation is used, if a reflection index of the light at the interface between the resist film and the antireflection film is too low, degradation may be caused on the sectional configuration of the patterned resist film. Namely, if the resist film is the positive type, the sectional configuration of the patterned resist film becomes a tapered form or have non-uniformly spread baseboard portion. On the other hand, if the resist film is the negative type, the sectional configuration of the patterned resist film becomes a reversed taper form or is formed into a shape having non-uniform cut-outs from the surface of the baseboard portion toward inside.
Accordingly, when the chemical amplification type resist film for far ultraviolet radiation is employed, even if the reflection index of the light at the interface between the resist film and the antireflection film is made small, the patterned resist film with satisfactory sectional configuration cannot be formed. It should be noted that the satisfactory sectional configuration means that the wall surface of the patterned resist film lies perpendicularly to the substrate.
When the first to third prior art set forth above is applied for the chemical amplification type resist film for the far ultraviolet radiation, the following problems should be encountered. FIG. 4A is a section showing a shape of the chemical amplification type resist film for the far ultraviolet radiation after development, and FIG. 4B is a plan view thereof. As shown in FIGS. 4A and 4B, an insulation layer 202d is formed on the surface of the silicon substrate 201d. An undercoat layer 216 as a film to be patterned is formed on the surface of the insulation layer 202d. Also, an antireflection film 221d of silicon nitride is formed on the surface of the undercoat layer 216. A positive chemical amplification type resist film (not shown) for far ultraviolet radiation is formed on the surface of the antireflection film 221d. After exposure of this resist film, it is developed to form the patterned resist film 233.
As shown in FIGS. 4A and 4B, when the chemical amplification type resist film for far ultraviolet radiation is formed on the surface of the antireflection film 221d of silicon nitride and is developed, if the condition is not appropriately selected, the baseboard portion of the patterned resist film 233 may non-uniformly spread. Accordingly, the shape of the baseboard portion 235 on the interface 251 between the patterned resist film 233 and the antireflection film 221d cannot become straight. Namely, a pattern width 236 defined by the shape of the baseboard portion 235 of the patterned resist film 233 cannot be a constant value.
As a result, even when the undercoat layer 216 is patterned with employing the patterned resist film 233 as an etching mask, the undercoat layer 216 cannot be removed in the desired width by etching. Thus, characteristics of the semiconductor device may be degraded.